The field of dilatometry has had a long and untilled need for a cost efficient and accurate dilatometer system, particularly for use in measuring various properties of materials such as quartz, glass, and the like. The coefficient of thermal expansion for these and other materials can be measured using a Fizeau interferometer. In this device, laser light interferes between the test surface and a flat reference surface, creating interference fringes. These fringes, in the shape of concentric rings known as Newton rings change as the temperature of the test material varies. Very small changes in the size of the test material, in the order of one angstrom, become visible.
One particular use to which this technology is applied is physical analysis of a variety of optical glasses. Each glass composition has its own specific set of properties, of course, and various end uses will determine the specific property in which an interest exists.
One of the more important physical characteristics of optical glass is the coefficient of thermal expansion. This property, called CTE, is defined as the amount that the material expands or contracts during a change in temperature.
In this field of optical dilatometry, in which properties are measured very accurately the prior art has emphasized the use of interferometers which employ collimated or parallel light. These systems have significant deficiencies in cost and accuracy. For example, since a reference mark of the viewing screen is used, accuracy of measurements depend on the stability of the interferometer and the test sample with respect to the reference mark on the viewing screen. To develop systems with maximum stability, would be of prohibitive cost.
It has recently been discovered that the CTE of optical glass may be measured accurately using apparatus and a method disclosed in a commonly owned application by Ralph T. Berg, for A METHOD AND APPARATUS FOR MEASURING COEFFICIENT OF THERMAL EXPANSION, now U.S. Pat. No. 4,989,980. The disclosure of this patent is incorporated herein by reference.
The method disclosed in the Berg patent includes the steps of generating an interference pattern defined by Newton rings which are a function of the specific material being examined. The area of at least one Newton ring is measured, and then the magnitude of change is the dimension is determined as a function of changes in area of that Newton ring. When temperature is used to change the dimension, a coefficient of thermal expansion can be calculated. A scale factor is determined which is a function of the difference between the area of a pair of successive Newton rings and of the wavelength of the laser beam.
The CTE calculations are done by following these steps. The area difference (A1-A2) for a predetermined Newton ring is calculated over a corresponding temperature difference (T2-T1). The area is then converted to a test sample length difference by multiplying by a scale factor. If the test sample has a unity length, e.g., a length equal to the unit of measure being used, the CTE is defined by the equation: ##EQU1##
The scale factor which is used to convert the area differences to a test sample length difference is derived based upon the knowledge that the difference between any two adjacent Newton rings is a constant which is proportional to one half of the wavelength of the laser beam. The scale factor is thus defined by the equation: ##EQU2##
The scale factor K is useful for calculating length differences by the equation: EQU Length difference=K (area at T2-area at T1).
As can be seen, the measurement of the area of the Newton rings is most difficult to measure precisely. These rings are not perfect circles with easy to measure areas. In the Berg patent, successful measurements were made by hand. Even when video camera images of the Newton rings were taken, so that precise measurement could be done at leisure, measurement of the area has not been easy or error free. There are means to measure the area of enclosed curves, but these methods are not well suited to Newton ring measurement, particularly when derived as shown in the Berg patent.
Video images of the Newton rings which have been created by interference of laser light between a test surface and a flat reference surface are often weak, just as the rings themselves are weak. In some instances, calculations for one single test has taken up to eight hours of time. It becomes a laborious process of manual data reduction as measurements are taken from a video monitor using the best available measuring equipment.
Development of an algorithm by which the area of a circle could be measured has not met with success. Efforts to locate the center of the circle were made difficult because of variations in instrument hardware. More importantly, precision measurement in the order of 1/10 part per million is sought, and algorithms based on the true or exact center of a circle will produce error since fringes from these Newton rings are not in fact true circles.
Other methods which have failed are those where the edge of the circle is followed. In this case, the algorithm fails when the edge does not return to the starting point, since these are not continuous rings. It should be noted that a fringe or Newton ring is rejected not because it is out of round but because it is discontinuous.
Accordingly, it would be a great advantage in the art to provide a method and apparatus for measuring the areas of Newton rings and other images and the like. It is also desirable to provide a method and apparatus which operates in cooperation with images on video screens. A great improvement in CTE measurement would be achieved if it were possible to substantially shorten the time needed to process data from a dilatometer and permit extremely accurate calculation of CTE and other physical information.